Glass tubing can be manufactured by a continuous process or by a discontinuous blowing (insufflation) process. The present state of the art allows drawing tubing up to a diameter of about 300 mm. Larger nominal widths can only be made by blowing into molds. The length of the glass tubing sections made by this method is determined by that of the mold, so that longer glass tubing must be made by fusing several such sections together.
The most diverse industrial applications require to manufacture tubing with the closest possible inside diameter tolerances. However the tolerances relating to glass tubing obtained both by drawing as well as by blowing are substantial, approximately +/-1 mm for lesser diameters and about +/-5-6 mm for larger ones. Where tight tolerances are desired, reshaping in the manner initially cited is required. There are several such methods:
The so-called KPG process is appropriate for reshaping sections of glass tubing with small diameters of a few millimeters as well as for dimensions as high as 200 mm. A non-scaling (non-oxidizing) metallic core is inserted in one glass tube section which is then closed at both ends. The inside space is evacuated through an aperture. A heat source raises the temperature of the glass until it deforms plastically and becomes snug with the core. Because of the differences in thermal coefficients of expansion, the metallic core shrinks more than the glass, whereby breaking off one end allows easy removal of the metallic core, and well satisfactory calibrated glass tubing section is thus obtained.
In another process, which is used mainly for lesser tubing diameters up to 30 mm maximum, the glass tubing is heated and continuously drawn over a mandril. The tolerance of the tubing thus made is somewhat larger than that for the KPG process.
All these processes suffer from the particular drawback of being only suitable for calibrating lesser diameter tubing. For larger rated widths, there has been so far no feasibility of calibrating the insides and outsides.
The invention addresses the problem of creating both a process and equipment for calibrating glass tubing sections even of appreciable and large diameters.
This problem is solved in that for a process of the initially cited kind, the glass of the tubing section clamped at both ends softened in the particular shaping zone in the case of external calibration is pressed against an external calibrating surface by means of a shaping element located inside said section and helically guided with respect to it, and in that for internal calibration, the softened glass is pressed against an internal calibrating surface by means of a shaping element outside said glass tubing and helically guided with respect to it.
Equipment for the implementation of the above problem and process is furthermore proposed, which is characterized by two mutually opposite, synchronously rotating clamping systems mounted on the same axis for the ends of the glass tubing section, by a holding means which is displaceable for both the inside and the outside of the glass tubing section and comprising a shaping element or a calibration surface, by an axially displaceable heater and by a mechanism allowing synchronous displacement of the holding means for the shaping element and calibration surface and the heater.
Using the process and the equipment of the invention, it is possible to carry out both an inside and an outside calibration of high precision on sections of glass tubing, adequate for the desired purpose. Tolerances of +/-0.2 mm were achieved for glass tubing sections with diameters as high as 450 mm.
Especially in recent years, glass apparatus design has increasingly required joints with other materials. Frequently for instance a column apex made of industrial glass must be provided with inserts of corrosion-proof metals. These inserts as a rule must be made hermetic in the column apex, which inevitably led to difficulties for the previously poor tolerances.
According to the invention, the glass tubing section clamped at both sides is merely softened in its particular zone of deformation. This can be implemented by any suitable heating system, for instance by a burner with its flame pointed at the particular location.
The helical guidance of the shaping element can be achieved while the glass tubing section is upright, however, preferably it will be rotated about its longitudinal axis and the shaping element will be guided axially along said glass tubing section. The shaping element and/or the calibration surface preferably consist of rotating rollers mounted axially parallel to the glass tubing section, even though other devices operating similarly may also be employed. The shaping element for instance may also be a convex surface. The calibration surface especially may also consist of a cylinder that in the case of rotating glass tubing sections may rotate concurrently. When two rollers are used as the shaping element or as the calibration surface, the latter will be wider and shall project beyond the former at both sides.
In a preferred procedure, the heat source--preferably a burner--will be guided synchronously with the shaping element and the calibration surface and parallel to these.